1. Field of Invention
The invention relates to devices and methods for transferring an image formed using liquid image development to a receiving medium or an intermediate transfer member.
2. Description of the Related Art
A typical electrographic printing device, such as, for example, a photocopier, a laser printer, a facsimile machine or the like, employs a uniform electrostatically-charged surface of a photoreceptor. The surface of the photoreceptor is exposed to a light beam, which is modulated according to image data that is to be printed. Exposing the surface of the photoreceptor to the light beam selectively discharges the electrostatic charge to form a latent image of the image data. The latent image is then developed by bringing a developer into contact with the latent image on the surface of the photoreceptor. The developed image, now recorded on the surface of the photoreceptor, is transferred to a substrate, such as, paper, either directly or indirectly, through an intermediate transfer. After transfer, the developed image on the substrate may then be subjected to further processing to fuse or fix the developed image to the substrate.
As is well known in the art, a latent electrostatic image can also be produced by image-wise applying electrons or ions to a dielectric surface. While this invention will be described primarily in terms of electrophotographic printing devices using photoreceptors, uniform charging, and image wise light exposure, the methods and claims of this invention apply equally to electrographic printing devices using dielectrics and iconography or electrography.
Two types of developers are generally used in electrographic printing devices: a dry developer, comprising toner particles, and perhaps carrier granules to which the toner particles electrostatically adhere; and a liquid developer, comprising electrostatically-charged toner particles dispersed within a carrier fluid. This liquid developer is also called a liquid toner.
When transferring an image developed with liquid toner, that is, transferring a liquid toner image, from the surface of a photoreceptor or intermediate transfer member, such as, for example, a belt, to the substrate, the completeness of the transfer depends, in part, on the amount of fluid between the liquid toner image and the substrate. For example, if there is insufficient fluid to fill the gap between the liquid toner image and the substrate, then not all of the portions of the developed image will transfer to the substrate. This gap may be caused by air bubbles between the liquid toner image and the substrate, the roughness of the substrate and the like. The resulting developed areas of the substrate where the toner was not transferred are called microvoids. The microvoids are the small white spots sometimes seen within an otherwise developed area. The problem of microvoids is well know in the art. Their relation to paper properties has been described in xe2x80x9cEffects of Paper Properties on Liquid Toner Transfer,xe2x80x9d by E. Caruthers et al., ISandTs NIP 15:1999 International Conference on digital Printing Technologies, pages 642-645 (Caruthers 1), incorporated herein by reference in its entirety.
Effectively transferring all of the toner particles of a developed portion of the image to the surface of the substrate may require additional fluid to completely fill the gap between the surface of the photoreceptor and the surface of the substrate. When the substrate is rough, that is, when the surface of the substrate is characterized by microscopic peaks and valleys, to avoid microvoid formation, the thickness of the fluid layer on the surface of the photoreceptor should be adequate to assure sufficient liquid toner transfer to fill all the microscopic surfaces of the substrate. The roughness of the substrate surface may be measured by observation through a microscope, by optical interferometry, or by measuring the movements of a stylus dragged over the surface. Typical roughness values, which reflect the distances between peaks and valleys of the substrate, may range from several microns to tens of microns. If the substrate is porous, extra fluid must be provided to compensate for wicking, that is, fluid removed from the surface of the substrate by the capillary action of the pores of the substrate. The porosity of the substrate may be measured by air bleed through the substrate, in units of time per volume of air, or by the absorption rate of fluid into the substrate, in units of volume of fluid per unit of time.
Various methods have been used to supply the necessary amount of fluid for the image transfer process. For example, fluid may be applied by pre-wetting the surface of the substrate, as disclosed in U.S. Pat. No. 4,358,195 to Kuehnle et al. However, pre-wetting a porous substrate greatly increases the amount of fluid, e.g., carrier fluid, applied because the carrier fluid wicks into the substrate during the time period prior to the image transfer. Furthermore, if the developed image covers only a limited area of the substrate, then pre-wetting the entire surface of the substrate uses more fluid than is necessary to transfer the developed image. Complete transfer of the liquid toner image should occur before wicking removes too much fluid from the surface of the substrate, or microvoids will likely form.
When the liquid toner image is pressed between the surface of a moving photoreceptor, such as, the rotating surface of a photoreceptor drum, and the moving surface of a substrate at, for example, a roller image transfer station, fluid shear forces may be produced. These fluid shear forces may cause image smearing in the direction of photoreceptor motion, including toward the trailing edge of the moving substrate, especially if the developed image is not particularly cohesive. These shear forces are especially likely to cause smear if the substrate surface is smooth and/or nonabsorbent. The problem of image smear is well known in the art. Its relation to paper properties is also described in Caruthers 1. To reduce this image smearing, developed images may be blotted or excess fluid may be removed by vacuum, such as provided in U.S. Pat. No. 5,332,642, which is incorporated herein by reference in its entirety. This blotting and fluid removal may compact the thickness of the developed image by removing excess carrier fluid and improve the cohesiveness of the liquid toner particles which form the developed image and reduce image smearing. However, removing too much carrier fluid could also increase the number of microvoids found within the transferred image.
This invention provides image forming methods and systems that apply a controlled amount of fluid to a movable image carrying member of an electrographic printing system that carries a developed liquid toner image before the developed liquid toner image is transferred to a substrate.
The amount of fluid applied may be based on the roughness and the porosity of a substrate onto which the developed liquid toner image is to be transferred. Re-wetting the developed liquid toner image on a photoreceptor instead of pre-wetting the substrate reduces the amount of re-wetting fluid, e.g., carrier fluid, that can wick into the substrate before transfer of the image to the substrate is complete. This reduces the total amount of fluid carried out of the system by the substrate and the amount of fluid which must be removed later by the fuser, and/or reclaimed and and/or returned to the toner supply.
The amount of fluid applied to the movable image carrying member may be controlled, for example, by a re-wet roller, located between a toner station and a transfer station. The re-wet roller may be movable closer to and farther from the moveable image carrying member. The rotational speed and direction of the re-wet roller can also be controlled. The image system may include a blotter/vacuum station between the toner station and the re-wet roller to remove fluid from the developed liquid toner image. The re-wet roller may be electrically charged to the same charge as the toner particles of the developed liquid toner image to repel toner particles from the re-wet roller. The action of the re-wet roller is generally similar to the action of a metering (or reverse) roller often included in the development system of a liquid toner printing device. Some effects on the final fluid layer thickness of roller-to-photoreceptor gap, process speed, and roll speed have been documented in the paper, xe2x80x9cReverse Roll Effects in Liquid Toner Electrophotography,xe2x80x9d by E. Caruthers et al., ISandT""s Eighth International Congress on Advances in Non-Impact Printing Technologies (1992), pages 206-208 (Caruthers 2), incorporated herein by reference in its entirety.
The image forming systems according to this invention may also utilize observational inputs from a user on the print quality to predetermine the amount of fluid to apply to the image carrying member. Alternatively, or additionally, in various exemplary embodiments the user may input data to a controller about the substrate type, or about the roughness and porosity of the substrate to control the amount of fluid applied to the image carrying member. In various exemplary embodiments, the image forming systems may also operate automatically by obtaining roughness and porosity data from the substrate using sensors within the image forming system and use the measured data to control the amount of fluid applied to the image carrying member.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.